The present invention relates to membrane compositions for the separation of components from fluid mixtures or solutions, and in particular to methods for preparing such membranes.
Separation processes such as dialysis, ultrafiltration, and reverse osmosis have been used in the separation of a wide variety of impurities and components from solutions. Membranes have been developed and used, for example, in the desalination of brackish and saline waters. Membranes have also been developed for the separation of gases. Membranes have been used for the separation of acid gases from methane, as described in U.S. Pat. Nos. 3,534,528; 4,130,403 and 4,561,654, or the separation of other gas mixtures as described in U.S. Pat. No. 4,230,463.
The properties of membranes can generally be defined in terms of permeation selectivity for specific components to be separated, permeation flux for one of the components to be separated, and mechanical strength of the membrane composition. The permeation flux is a measure of the rate at which one of the components to be separated permeates through the membrane. Permeation selectivity is an indication of the ratio of the permeation fluxes of the components to be separated, often referred to as the separation factor. Permeation flux is generally inversely proportional to the thickness of the discriminating layer. Advantageously, the discriminating layer is as thin as possible. Unfortunately, extremely thin membrane structures are typically fragile, have poor mechanical strengths and frequently contain discontinuities, small holes or other defects.
Known membrane compositions include cellulose ester membranes, which are typically employed in flat sheet or hollow fiber form. The so-called Loeb-Sourirajan method for preparing such membranes involves providing a microporous structure for support and a dense layer for separation. This method produces asymmetric membranes which possess good physical strength and membrane characteristics. Unfortunately, the method involves process limitations which are difficult to control and, as noted in U.S. Pat. No. 4,430,807, can require special drying techniques if used for gas separation.
Numerous attempts have been made to improve both the membrane compositions and the processes for their preparation. For example, Henis et al., Science, Vol. 220, pg. 11 (1983) disclose a gas separation membrane composition which comprises a coating of silicone rubber in an attempt to plug flaws and defects present in the membrane. Unfortunately, such membrane compositions normally exhibit permeation selectivities intermediate between the silicone rubber and the remainder of the membrane. Delamination of the coating can also occur under certain operating conditions.
Attempts to provide solvent crazed polystyrene microporous membranes by Michaels et al., Advances in Polymer Science, Vol. 27 (1978) have proved unsuccessful. In particular, such membranes exhibit a small increase in flux and are too weak for practical uses.
In view of the deficiencies of the art, it would be highly desirable to provide a membrane composition and a process for preparing same, which membrane composition exhibits good permeation flux and selectivity, good mechanical strength, and which can be prepared in an efficient and cost-effective manner.